In the case of a porcelain insulator, different properties – such as, protection from
environmental impact and provision of sufficient creepage distance on the one hand,
and mechanical strength on the other – are united in a single component. In an arrester
with polymer housing, however, these properties are apportioned to two different components.
Mechanical strength is, virtually without exception, achieved with fiber-glass
reinforced plastic (FRP) materials. In the example shown in Figure 15, several rods
serve this purpose. They are strained, e.g. by crimping, in the end fittings and enclose
the MO resistor stack like a cage. That is why the term "cage design" is used in this
context2. Here, the MO resistors themselves form part of the mechanical structure. This
is how a mechanical high-strength unit out of MO resistors, end fittings and the FRP
structure are created. This module is inserted in a mold, in which silicone rubber is directly
injected. With the appropriate manufacturing techniques, it is possible to obtain a perfect bond of the silicone rubber with the other components, void-free and permanent.
The final product is all of a piece. One advantage of the applied silicone rubber in this
case, in comparison to cheaper materials, which are also used, is the excellent endurance
properties – by now, it is possible to fall back on about 40 years of service experience in
this area. Another advantage is a characteristic unique to silicone rubber,
hydrophobicity: even if the silicone surface is heavily polluted, water simply drips off.
This suppresses the formation of conductive layers and advantageously affects the
operational performance of the arrester in polluted conditions.